Process for Forming Anticorrosive Coating

ABSTRACT

Anticorrosive coating is formed cheaply and in a short period, which facilitates its general application to marine steel structures. 
     A steel caisson  1  is used as a cathode, an undersea member  3  arranged in seawater and opposed to the steel caisson  1  is used as anode. Direct current is passed between the electrodes so that anticorrosive coating  8  is formed on the steel caisson  1  through electrolytic reaction of the seawater, thereby attaining anticorrosion. The electric current is passed between the electrodes such that coating  7  with magnesium hydrate as dominant constituent is formed on the steel caisson  1  to have a predetermined thickness; then, supply of the electric current is stopped. Thus, anticorrosive coating  8  is formed through compositional substitution effect of substituting calcium carbonate for the magnesium hydrate in the presence of seawater.

TECHNICAL FIELD

The present invention relates to a process for forming anticorrosive coating and more specifically to a process for forming anticorrosive coating on a marine steel structure in a short period.

BACKGROUND ART

A process for anticorrosion of a marine steel structure has been proposed in which the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to the steel structure. Direct current is passed between the electrodes to form coating (anticorrosive coating) on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the steel structure.

Thus, Reference 1 discloses that a steel member constituting a surface of a marine steel structure is used as a cathode, an anode being arranged in seawater to be opposed to the steel member. Direct current is passed between the electrodes to remove rust and the like scales on the surface of the steel structure. Then, direct current is passed between the electrodes to deposit electrodeposit, which has electrolytic reaction product of the seawater as dominant constituent, on the surface and any corroded pores of the steel structure, thereby forming anticorrosive coating.

[Reference 1] JP10-313728A SUMMARY OF THE INVENTION Problems to be Solved by the Invention

The coating formed as mentioned above on the marine steel structure through electrolytic reaction of the seawater has calcium carbonate CaCO₃ and magnesium hydrate Mg(OH)₂ as main components. As is known in the art, it is calcium carbonate formed hard in hardness that exhibits anticorrosive effect. Therefore, in order to make a marine steel structure anticorrosive, anticorrosive coating having calcium carbonate as dominating component must be formed on the steel structure.

As disclosed also in Reference 1, relationship between coating composition on a marine steel structure and current density due to electric current applied between electrodes is as shown in FIG. 1. More specifically, in a condition of low current density, composition ratio of calcium carbonate in the coating is high and that of magnesium hydrate is low. As the current density is elevated, composition ratio of calcium carbonate is decreased and that of magnesium hydrate is increased. It is regarded that good anticorrosion property is obtained when coating composition ratio of calcium carbonate to magnesium hydrate is 1 or more.

So, it is disclosed in Reference 1 that current density during electrodeposition is selected within a range of 0.2 to 2 A/m² (1 A/m² on an average) so as to form anticorrosive coating with calcium carbonate as dominant constituent and that anticorrosive coating in the form of hard electrodeposition coating and with thickness of about 5 mm or more is formed on the marine steel structure.

However, it is known that a long period of about 10 months or more is required for formation of anticorrosive coating with calcium carbonate as dominant constituent and having thickness of for example 5 mm or more by a low current density of for example 1 A/m² as disclosed in Reference 1.

Thus, the conventional process for forming anticorrosive coating as disclosed in Reference 1 requires long construction period and long-term management and is costly due to increased electric power consumption, so that the process has not been practically applied except special cases such as bridge piers at sites in deep water or in violent tidal current.

The invention was made in view of the above and has its object to provide a process for forming anticorrosive coating which can be conducted cheaply in a short period, thereby easily leading to general application to marine steel structures.

Means or Measures for Solving the Problems

The invention is directed to a process for forming anticorrosive coating on a marine steel structure wherein the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to said steel structure, direct current being passed between the electrodes, anticorrosive coating being formed on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the marine steel structure, characterized by passing the electric current between the electrodes so as to have current density to form coating having magnesium hydrate as dominant constituent on said marine steel structure, thereby forming the coating with a predetermined thickness, then stopping supply of the electric current to thereby provide anticorrosive coating through compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate.

It is preferable in the above-mentioned invention that the electric current is passed between the electrodes so as to attain current density of the marine steel structure in a range of 3 to 10 A/m².

Effects of the Invention

According to a process for forming anticorrosive coating of the invention, electric current is passed between electrodes so as to keep high current density of a marine steel structure, so that coating with magnesium hydrate as dominant constituent is formed on the steel structure in a short period. Then, supply of the electric current is stopped to utilize a compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate, thereby forming anticorrosive coating. As a result, the invention has an effect that it can form good anticorrosive coating with calcium carbonate as dominant constituent in by far a shorter period than ever before.

Such formation of anticorrosive coating in a short period leads to reduction in construction period and easiness in management as well as reduction in cost due to reduced electric power consumption. Therefore, the invention has an effect that it can be easily applicable to any kind of marine steel structures unlike the conventional process with limited applicability to special sites.

BRIEF DESCRIPTION OF THE DRAWINGS

[FIG. 1] A diagram showing relationship between coating composition on a marine steel structure and current density due to electric current applied between electrodes.

[FIG. 2] A side view exemplifying equipment components in application of a process for forming anticorrosive coating according to the invention to a steel caisson of a breakwater which is an example of a marine steel structure.

[FIG. 3] A front view looking in the direction of arrows III in FIG. 2.

[FIG. 4] A diagram showing test results of generated amount of calcium carbonate.

[FIG. 5] A diagram showing test results of generated amount of magnesium hydrate.

[FIG. 6] A view showing coating generated on a cathode substrate in the test equipment.

[FIG. 7] A view showing change of the coating in FIG. 5 into anticorrosive coating through compositional substitution effect.

[FIG. 8] A diagram showing variation in composition ratio of magnesium hydrate and calcium carbonate by stopping supply of electric current after formation of coating with magnesium hydrate as dominant constituent.

[FIG. 9] A diagram showing variation of the coating amount on a mol basis.

[FIG. 10] A diagram showing variation of the coating amount on a weight basis.

EXPLANATION OF THE REFERENCE NUMERALS

1 steel caisson (marine steel structure) (cathode)

2 DC power supply

3 undersea member (anode)

7 coating

8 anticorrosive coating

BEST MODE FOR CARRYING OUT THE INVENTION

An embodiment of the invention will be described in conjunction with the attached drawings.

EMBODIMENT 1

FIG. 2 is a side view exemplifying equipment components in application of a process for forming anticorrosive coating according to the invention to a steel caisson of a breakwater which is an example of a marine steel structure; and FIG. 3, a front view looking in the direction of arrows III in FIG. 2. In the figures, reference numeral 1 denotes steel caissons which constitute a breakwater; 2, a DC power supply on arranged for example on a top of the steel caisson 1; and 3, undersea members suspended in seawater to be opposed in a predetermined distance to the submerged surface of the steel caisson 1 and spaced from each other by a predetermined distance and in parallel with the surface of the steel caisson 1. The undersea members 3 may be made from soluble material such as magnesium or aluminum or insoluble material such as titanium.

The DC power supply 2 is connected at its minus (−) side to the steel caisson 1 so as to use the steel caisson 1 as a cathode and is connected at its plus (+) side to the undersea member 3 so as to use the undersea member 3 as an anode. Such construction is made to each of the plural steel caissons 1.

A predetermined constant current is passed between the electrodes, i.e., between the steel caisson 1 and the undersea member 3 by the DC power supply 2, so that deposited coating is formed on the steel caisson 1 through electrolytic reaction of the seawater.

Moreover, as shown in FIG. 3, monitoring electrodes 4 are arranged at plural points on the submerged surface of each steel caisson 1, a monitoring unit 5 being arranged for example on the top of the steel caisson 1 so as to determine and display electric potential from detected values of the respective monitoring electrodes 4. The monitoring unit 5 severs for checking that electric current with a predetermined current density is passed through the steel caisson 1 through application of constant electric current on the steel caisson 1 by the DC power supply 2. At sites with great tidal variation where an area of a submerged surface on the steel caisson 1 varies greatly to greatly change the current density of the electric current passed through the steel caisson 1, there may be employed a constant-potential system which keeps constant the electric potential (voltage) of the steel caisson 1. In this case, the monitoring unit 5 may have the function of a controller for automatically controlling the voltage of the DC power supply 2 so as to keep the detected electric potential to be a predetermined constant potential.

Mode of operation of the invention will be described in conjunction with experimental example.

Using the equipment shown in FIGS. 2 and 3, the inventors tested generated amounts of calcium carbonate and magnesium hydrate, respectively, when the current density passed through the steel caisson 1 is gradually varied with the current-carrying capacity through the DC power supply 2 being constant or 60 A.h/m . The test results of the generated amounts of calcium carbonate and magnesium hydrate are shown in FIG. 4 and in FIG. 5, respectively.

FIG. 4 shows the fact that, as the current density is increased, the generated amount calcium carbonate is rapidly increased into its peak with the current density being 0.5 A/m² or so; as the current density is further increased, then the generated amount of calcium carbonate tends to be rapidly decreased.

FIG. 5 shows the fact that, as the current density is increased up to about 7 A/m², the generated amount of magnesium hydrate is increased toward its peak; as the current density is further increased, then the generated amount of magnesium hydrate tends to be decreased.

It turned out that the above-mentioned tendencies in FIGS. 4 and 5 coincide with the conventionally known relationship between current density and coating shown in FIG. 1.

Then, the inventors took notice of FIG. 5 to find out that with the current density being set to be as high as of 3 to 7 A/m², generating speed of coating can be substantially increased, so that coating with intended thickness can be formed in a short period. The data shown in FIG. 5 are those in laboratory experiment with still water condition, so that in actual sea areas, electrodeposition efficiency may be lowered due to effect of tidal current (see, for example, Honshi-Giho Vol. 24, No. 95 (December 2000)). As a result, tests were conducted with respect to generated amount of coating in real sea areas to find out that an optimum range of current density in real sea areas is 3 to 10 A/m².

However, with the current density being set to be as high as 3 to 10 A/m² as mentioned above, the coating formed on the steel caisson 1 has magnesium hydrate as dominant constituent (for example, 95% of magnesium hydrate), failing to be anticorrosive coating.

Thus, the inventors made researches so as to change the coating with magnesium hydrate as dominant constituent and formed in a short period as mentioned above into anticorrosive coating with calcium carbonate as dominant constituent and found out that compositional substitution effect occurs in seawater to substitute calcium carbonate for the magnesium hydrate, thereby utilizing such effect to attain formation of anticorrosive coating.

The inventors used experimental equipment similar to that shown in FIGS. 2 and 3 to conduct tests for forming coating so as to ascertain the above-mentioned compositional substitution effect.

In the experimental equipment, cathode substrate in opposed relationship to the steel caisson 1 is made from SS (stainless steel) 400 and the anode member corresponding to the undersea member 3 is made from Mg, using natural seawater with temperature of 25° C. with the current applying condition of current density being 3 A/m². Thus, coating test for 30 hours was conducted.

As shown in FIG. 6 with no submerged period, thickness L of the coating 7 generated on the cathode substrate 6 in the test was detected to be 105 μm. The composition obtained in chemical analysis of the coating 7 was, as shown in FIG. 8, about 5% of calcium carbonate and about 95% of magnesium hydrate, most of the composition being magnesium hydrate.

Then, tests were conducted to ascertain the compositional substitution effect by the coating 7 with magnesium hydrate as dominant constituent as mentioned above.

More specifically, after the coating 7 with magnesium hydrate as dominant constituent as mentioned above was formed on the cathode substrate 6, supply of electric current by the DC power supply was stopped. Thereafter, the coating was kept submerged in seawater; coating composition ratio of magnesium hydrate and calcium carbonate and coating amounts in g/cm² and in mmol thereof were detected at 7, 14 and 21 days after the start of submerging (the stop of current supply).

According to FIG. 8, as the submerging was started, the dominant magnesium hydrate was decreased and calcium carbonate was increased; it turned out that dominance in composition ratio between magnesium hydrate and calcium carbonate was reversed substantially at 17 days after the submerging.

With the lapse of submerged period, there was no variation in coating amount on the mol basis as shown in FIG. 9. On the other hand, there appeared slight increase tendency in coating amount on the weight basis as shown in FIG. 10. This tendency means change of the coating composition ratio due to decrease of magnesium hydrate and substitutive increase of calcium carbonate with the lapse of submerged period as shown in FIG. 8. More specifically, change of the coating composition ratio with no change of the total amount on the mol basis as shown in FIG. 9 is regarded to mean that dissolution and deposition reactions of magnesium hydrate and calcium carbonate, respectively, in the coating concurrently occurred with chemical equivalent of substantially 1:1. As a result, coating amount on the weight basis was increased as shown in FIG. 10 since calcium carbonate with higher molar weight (Mw=100) was substituted for the magnesium hydrate with lower molar weight (Mw=58).

More specifically, it is regarded that the following reaction formula (1) occurs to bring about the reaction formula (2), leading to the reaction formula (3).

Mg(OH)₂→Mg²⁺+2OH⁻  (1)

Ca²⁺+H₂CO₃+2OH⁻→CaCO₃+2H₂O  (2)

Mg(OH)₂+Ca²⁺+H₂CO₃→Mg²⁺+CaCO₃+2H₂O  (3)

Thus, the coating 7 with magnesium hydrate as dominant constituent as shown in FIG. 6 is replaced by calcium carbonate through the above-mentioned compositional substitution effect to thereby form, as shown in FIG. 7, the hard anticorrosive coating having calcium carbonate as dominant constituent with no substantial change in thickness L.

As can be seen from FIG. 8, submerging of 17 days or more brings about excellent anticorrosive coating 8 with coating composition ratio of calcium carbonate to magnesium hydrate being 1 or more.

In the experimental tests shown in the above, the electric current applying condition was the current density of 3 A/m². That is, the experiments were conducted with lower current density since the coating tends to fall off in the case of the experimental equipment with no flows of seawater. However, it turned out that in actual natural seawater with flows, coating can be formed well with no falling-off even if the process is conducted with the current density of as high as 3 to 10 A/m².

Thus, by contrast with the conventional process having a period of 10 months or more in formation of anticorrosive coating, anticorrosive coating according to the invention can be formed in a very short period on the order of one month or one month and a half. This facilitates the process management and brings about reduction in electricity consumption, resulting in decrease in cost. Therefore, with no limitation to the special sites unlike the conventional process, the invention can be easily applied to any kinds of marine steel structures.

When a process for forming anticorrosive coating according to the invention is applied to an existing marine steel structure and where any extraneous matter such as rust on a surface of the marine steel structure is required to be removed beforehand, various countermeasures may be employed such as application of direct current between electrodes for removal of rust and the like, removal through injection of high-pressure water or removal through manpower.

It is to be understood that the invention is not limited to the above embodiment and that various changes and modifications may be made without departing from the scope of the invention. For example, the description of the embodiment has been made with respect to steel caisson as an example; however, the invention may be similarly applicable for anticorrosion of various marine steel structures such as steel sheet pile or steel bridge pier. 

1. A process for forming anticorrosive coating on a marine steel structure wherein the steel structure is used as a cathode, an anode being arranged in seawater to be opposed to said steel structure, direct current being passed between the electrodes, anticorrosive coating being formed on the steel structure through electrolytic reaction of the seawater, thereby attaining anticorrosion of the marine steel structure, characterized by passing the electric current between the electrodes so as to have current density to form coating having magnesium hydrate as dominant constituent on said marine steel structure, thereby forming the coating with a predetermined thickness, then stopping supply of the electric current to thereby provide anticorrosive coating through compositional substitution effect which occurs in the presence of the seawater to substitute calcium carbonate for the magnesium hydrate.
 2. A process for forming anticorrosive coating as claimed in claim 1, characterized in that the electric current is passed between the electrodes so as to attain current density of the marine steel structure in a range of 3 to 10 A/m². 